Electron beam accelerators are used to irradiate products with a beam of electrons. This form of irradiation has a number of useful applications. Medical products such as sutures, syringes, gloves, and other packaged items can be sterilized by this means. Electron beam irradiation can pasteurize and sterilize food products as well as de-infest them of insects and parasites. Electron beam irradiation can also be used to alter material properties such as the polymerization, curing or enhancing of polymers, plastics and cellulose.
The electron beam accelerators used for these applications are high power accelerators. The current industry standard is to employ an accelerator having a beam energy of 10 MeV. The depth of penetration of the electron beam is proportional to the electron beam energy and the density of the product. As a result, the surface density of the product must be limited to approximately 3.5 g/cm.sup.2 for single sided radiation and approximately 8 g/cm.sup.2 for a product being irradiated from opposite sides.
When sterilizing and pasteurizing products, it is essential that the electron beam sufficiently penetrate the product in order that it deliver the prescribed radiation dose to all areas of the product being irradiated. Correspondingly, there must also be an effective means to measure the absorbed dose to ensure that adequate irradiation has taken place.
The current practice used to measure absorbed dose is to qualify a particular product for electron beam irradiation in order to establish the efficacy of the irradiation process on that product. The absorbed dose is measured by plating film dosimeters at various locations throughout the product, irradiating the product and then reading the dosimeters and tabulating the dose at each dosimeter location. The radiochromic dosimeters acquire an optical density in proportion to the radiation dose that they receive from the electron beam. From the tabulations of minimum and maximum dose, the nominal dose is established and the packing and orientation of the product is then specified to achieve the required dose. By this method, the qualification of each product creates a recipe for irradiation of that product. For the routine irradiation of subsequent production quantities, quality assurance procedures are followed to ensure that the recipe is followed. One commonly adopted quality assurance procedure is to place a dosimeter on the outside of a sample product box to confirm that the box passed through the electron beam. The placement of dosimeters inside product boxes prior to the irradiation process is precluded because retrieval of the dosimeters after irradiation would recontaminate the product.
There are a number of drawbacks to the method currently being used. For instance, if the product is incorrectly loaded on the irradiation conveyor or tray (for example it may be placed on its edge instead of on its bottom), the irradiation may be ineffective if the exposed surface of the product has too great a surface density to allow penetration of the product. Ineffective irradiation caused by incorrect loading is difficult to detect with dosimeters placed on the outside of product packages. Further, as it is impractical to place dosimeters on the outside of each and every product package, there is no assurance that a product not tested has been adequately irradiated.
Another potential problem arises when product manufacturers either deliberately or inadvertently repackage products or vary the composition of the product without informing the operator of the irradiation service centre. The operator is not normally permitted to open the manufacturers' boxes, and will thus fail to recognize that the new product either requires a new irradiation recipe or that it cannot be properly irradiated. For example, if an irradiation service centre qualifies the sterilization of a box of rubber gloves and the glove manufacturer subsequently discovers a way to compress the gloves and package more gloves into each box, without informing the service centre operator, the new box may be too dense to be penetrated by the electron beam. Irradiation would be ineffective and the problem would go unnoticed.
Another disadvantage to the use of film dosimeters to detect absorbed dose is that they may not detect malfunctioning of the accelerator. It is possible that, through malfunction, the energy of the accelerator may change during irradiation of product. Accelerators which are able to accurately monitor their energy will shut down and avoid improper treatment of product, however, other accelerators rely on periodic calibration to verify beam energy. Thus, depending on the time between calibrations, a significant quantity of product may have to be held in quarantine until the energy is verified. If the energy is found to be out of tolerance, significant amounts of product may need to be discarded.
Thus there is a need for a more accurate and reliable method for instantaneously assessing the radiation dose absorbed by a product to ensure that it has been properly irradiated.